Dear Peter,

This surprising behavior is not a bug in the DXA and can be explained as follows:

You are dealing with a BCC crystal. Every cubic crystal lattice has 24 equivalent orientations. Thus, given a crystal model that has some orientation in space, you have 24 different ways of defining the axis orientations of the crystal, i.e., the spatial directions of the [100], [010] and [001] Miller indices.

When the DXA sees your input crystal, it will pick one of these 24 physically equivalent rotations at random. It's important to note that the algorithm newly picks the orientation in each simulation frame in an uncorrelated manner, because it doesn't remember which orientation it has used for the previous frame.

So what you are observing is a flipping of the crystal lattice orientation to a different symmetrically equivalent orientation. Even though the dislocation doesn't change in any, the Burgers vector displayed by the DXA apparently does, because it gets expressed in a newly oriented lattice coordinate system.

The column "Spatial Burgers vector" shows that the physical Burgers vector, which is obtained by transforming the calculated true Burgers vector from the lattice coordinate system to the simulation coordinate system, indeed does *not* change. [2.4739,0.00,0.00] and [2.4739,0.00,-0.00] are essentially the same spatial vector --small numerical errors aside that typically occur due to elastic distortions of the crystal.

The dislocation line changes color, because you selected to color the lines "by Burgers vector" and not "by dislocation type". In the latter mode, the line would probably stay green, because 1/2[111] and 1/2[11-1] belong to the same class of dislocations.

Note that the spurious flipping of the crystal orientation only occurs for certain alignments of the crystal lattice, like in your particular case where the <111> direction is aligned along the y-axis of the simulation coordinate system. For the DXA, this orientation is right on the edge of a standard orientation triangle. So even an infinitesimal reorientation of the crystal into an adjacent standard triangle, which is associated with a symmetry flip of the lattice. Other orientations, which are located more in the interior of the standard triangle, won't show this unstable behavior and DXA will typically maintain the same orientation over time.

-Alex